MXPA99007697A - Laser marking of articles - Google Patents

Abstract

A method of marking or forming a transparent window in an article (6), such as a security document, is provided in which areas of opacifying layers of printed ink (8, 10) on opposite surfaces of a polymeric film or substrate (12) are irradiated by laser radiation (4) of a selected wavelength. The opacifying layers (8, 10) have greater absorption characteristics for the laser radiation than the film or substrate (12) which is substantially transparent to the selected wavelength of the laser radiation so that after ablation of the area of printed matter (8) on one surface, the radiation passes through the substrate (12) substantially unaffected to ablate the area of printed matter (10) on the opposite surface. In the resultant article, the markings or windows formed by the areas of printing removed from each surface of the substrate (12) are in register, and provide a deterrent against counterfeiting.

Description

MARKING OF ARTICLES WITH LASER DESCRIPTION OF THE INVENTION The present invention relates to a method for marking articles with a laser and more particularly to a method for marking security documents or other documents having a transparent substrate covered by opacifying layers, such as printed matter. There are previously known methods for marking glass or plastic materials using a laser. A high-energy laser beam is made to converge in a plane below the surface of the material to form an opaque region through the ionization of the material at the point of convergence. In another known arrangement, the marking of plastic objects by a colored laser is performed by exposing a uered plate, which is applied to the surface of the object, and then laser radiation is applied to it. The film, which contains at least one color component, is softened in the irradiated areas and the color component penetrates the surface and the non-irradiated areas of the surface are dissolved by a suitable solvent to leave a darker impression where the mark is located. Security papers are also known, such as banknotes, checks and identity cards that have luminescent authenticity marks made with substances that only shine in the visible spectrum. The surfaces of laser-sensitive plastics, such as polyvinyl chloride (PVC), have been marked causing a change of color in the irradiated area of the surface. When a particular plastic is not sensitive to the laser, such as polypropylene (PP), polyerylene (PE) or polyethyltetraphthalate (PET), it can be made sensitive to the laser by incorporating a laser-sensitive additive into the plastic. Yet another arrangement discloses a lithographic printing plate, which can be laser printed, which has a first layer and a substrate layer below the first layer. The substrate has efficient absorption characteristics of infrared laser radiation, the first layer and the substrate layer each have different affinities for the ink (in a dry plate construction) or an adhesive fluid as ink (in a wet plate construction) . When irradiated by the laser, the substrate absorbs the radiation and ablation occurs on the surface of the substrate which is in contact with the first layer. This leads to a release of the substrate under the first layer which is then removed at the exposure point. The result of the removal is a printing stain whose affinity for ink or adhesive fluids differs from that of the first unexposed layer leading to a permanent mark. While the methods for marking articles described above are useful for marking a side or surface of an article, if it is desired to mark the opposite sides or surface of an article, two separate laser marking operations are required. Therefore, it is desirable to provide a simple and effective method for marking opposing surfaces of an article, such as a security document. It is also desirable to provide a convenient method for forming a transparent window in an article such as a security document. According to one aspect of the present invention there is provided a method for marking an article, the article comprises a substrate with opacifying layers on opposite surfaces of the substrate, said method comprising: irradiating an area of the opacifying layer on a surface of the substrate with laser radiation such that the area of the opacifying layer on such a surface undergoes ablation by laser radiation to produce a mark by removing the area of the opacifying layer on such a surface, where the laser radiation travels through the substrate and causes ablation in an area of the opacifying layer on the opposite surface of the substrate to produce a mark by removing the area of the opacifying layer from the opposite surface of the substrate. The present invention also provides a method for creating a transparent window in an article comprising a transparent substrate with opacifying layers on the opposite surfaces of the substrate, said method comprises: irradiating an area of the opacifying layer on a surface of the substrate with laser radiation in such a way that the area undergoes ablation by laser radiation to remove the area of the opacifying layer of such surface, wherein the radiation of the laser travels through the substrate and causes ablation in an area of the opacifying layer on the opposite surface of the substrate to create a transparent window in the substrate. Preferably, the opacifying layers on the opposite surface of the substrate have higher absorption characteristics for laser radiation than the substrate.

Preferably, the wavelength of the laser radiation is selected in such a way that when the article is exposed to radiation, the opacifying layers absorb the radiation and undergo ablation of the substrate, the substrate being substantially transparent to the radiation at that length of The wave selected in such a manner that the laser radiation travels through the substrate substantially unhindered. The laser radiation can be a continuous beam. Alternatively, it can be issued as a pulse or series of beats. The present invention also provides an article comprising a substrate having opacifying layers on opposite surfaces of the substrate, the substrate being formed of a material, which is substantially transparent to the laser radiation of a selected wavelength, wherein each of the surfaces has a mark formed within it, such marks are formed by removing a first area of the opacifying layer on a surface of the substrate forming an ablation in the first area with the laser radiation at the selected wavelength, and allowing the laser radiation passes through the substrate to remove one area of the opacifying layer on the other surface in accordance with the first area removed from the opacifying layer on that surface. This invention is generally related to printed articles, such as banknotes, security documents or any other document having matter disposed thereon, having a transparent substrate with opacifying layers of ink printed on the opposite surfaces of the substrate in at least a portion of the Article. The laser radiation can be used to create a transparent area, in the form of a particular design or character or symbol, in the portion of the article where there are layers of ink printed on one or both sides of the transparent base substrate. This is done by removing or creating the ablation of the printed ink layers of each or both sides of the substrate in the appropriate areas of the article. A certain relationship must exist between the wavelength of the laser light used and the relative absorption characteristics of the transparent substrate and the layers of printed matter at the selected wavelength. For example, the layers of printed matter should be good absorbers of the laser radiation in such a way that the layers can be removed or ablated when exposed to radiation for a predetermined period. Also, the transparent substrate should be a poor absorber of the laser radiation compared to the absorption characteristics of the printed layers at the selected wavelength. The substrate should be substantially transparent to radiation at this wavelength, allowing light at the selected wavelength to travel through the substrate without creating substantial damage or distortion to the transparent substrate. In this way, a transparent window is formed with both surfaces of the substrate being in accordance with the desired shape, symbol, design or character in the places of radiation exposure. The present invention is particularly, but not exclusively, applicable to banknotes or other security documents having at least a portion of the document formed from a transparent plastic substrate having at least one opacifying layer of ink on both surfaces. The transparent plastic substrate is preferably formed of a transparent polymeric material, such as PE, PP or PET, which can be made of at least one biaxially oriented polymer film. The substrate may comprise a single layer of film of polymeric material. Alternatively, the substrate may comprise a sheet of two or more layers of bi-axially oriented transparent polymeric film of the type described in Australian Patent NO. AU-A-87665/82, whose content is incorporated herein by reference. The opacifying layers of the printed matter may comprise one or more of a variety of opacifying inks which may be used in the printing of banknotes or other security documents. For example, the opacifying ink layers may comprise pigmented coatings comprising a pigment, such as titanium dioxide, dispersed with a binder or carrier of polymeric material that is crosslinkable as described in the specification of Australian Patent No. A- A-87665/82. Preferably, the selected wavelength of the laser radiation falls substantially within the range of about 0.5 microns to about 20 microns. In a particularly preferred embodiment, the transparent plastic substrate is formed of PP, the opacifying ink layers are formed of Ti02 and silica dispersed within a polyurethane-based resin, and the wavelength of the laser radiation used is of either about 1.06 microns or about 10.6 microns. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures 1 (a), (b) and (c) show a continuous laser beam acting on an article and creating the ablation on the printed matter of the surfaces of the article; Figures 2 (a), (b) and (c) show a laser emitting a single pulse of radiation and acting on an article to create ablation on the printed material of the surfaces of the article; Figures 3 (a), (b), (c) and (d) show a laser emitting a series of radiation pulsations acting on an article. to create ablation on the printed matter of the article surfaces. Referring to Figure 1, which illustrates an ablation process in an article using a continuous wave laser system. A laser source 2 emits a continuous beam 4 of a laser light incident on and acting on one side of an article 6. The article 6 has a first layer 8 comprising printed matter ink, a second layer 10 also comprising printed ink material and a transparent substrate in the form of a polymeric film 12 between layers 8 and 10. As shown in Figure 1 (a), the ray 4 initially makes contact with the first layer 8 leading to heat build-up located in the layer due to radiation absorption by layer 8 as shown in Figure 1 (b). In the end, the internal bonds and the cohesion forces of the layer structure weaken and break leading to ablation or removal of particles 14 from layer 8. In this step, laser beam 4 has penetrated the first layer 8 and travels through the film 12 substantially unhindered until it strikes the surface of the layer 10 located on the other side of the 'film 12. Little or nothing of the absorption of the radiation is carried out while the beam 4 travels through the film 12 and thus little or no heat accumulation and consequent damage to the film occurs. The difference in the absorption characteristics between the layers 8 and 10 and the film 12 is relative and therefore there may be some heat accumulation in the film. The laser beam 4 should, therefore, only be exposed to the article 6 for a sufficient period of time so as not to cause appreciable distortion or damage to the film 12. The relative absorption characteristics of the layers 8 and 10 and the film 12 and the energy output of the laser source 2 should be chosen in such a way as to provide the necessary ablation of the layers resulting in a transparent window. As shown in Figure 1 (c), when the laser beam 4 strikes the second layer 10 a similar ablation process occurs whereby the particles 16 are removed from the surface thereof as with the layer 8. This it leaves a transparent area which is in agreement on both surfaces of the film 12 in the form of the design or symbol, etc. wanted. Figure 2 depicts an ablation process in an article using a laser source with pulsations 20 in which a single pulse 18 of radiation as shown in Figure 2 (a) is emitted from the laser source 20 to impinge in article 6 on one side. As shown in Figure 2 (b) the laser pulse 18 has sufficient energy to traverse and create ablation of the surface of the first layer 28 resulting in the particles 26 of the layer being dislodged in the same manner with with respect to the process of Figure 1. Pulsation 18, with its remaining energy continues to travel through film 12 with little or no absorption of radiation, and therefore there is no heat buildup and subsequent damage to the film, until which affects the second layer 10 of the printed material. Because the pulsation has sufficient energy, an ablation process similar to layer 20 occurs in which particles 28 are removed from the surface of layer 22 as shown in Figure 2 (c). A transparent window results, in which the ablation areas of the layers 8 and 10 form the desired character, symbol or design on both surfaces of the film 12 which are in accordance. If the energy of the laser using a single pulse is not enough to create the ablation of the layers of the printed matter completely, then more than one pulse or a series of pulsations can be used as shown in Figure 3. These multiple pulsations and consecutive are emitted from a laser source with pulsations 20. The first pulse 30 emitted (see Figure 3 (a) and 3 (b)) impinges on the first layer 34 of the printed matter resulting in the particles 40 being removed, but they do not have enough energy to create ablation by. complete of the ink layers 34 and 38. A second pulse 32, and if necessary subsequent pulsations, are emitted from a laser source 20 as shown in Figure 3 (c) and the second pulse 32 and / or pulsations Subsequent, have or have sufficient energy to complete ablation of layer 34 and continue through film 36 substantially unhindered and create ablation of layer 38 and remove particles 42 as shown in Figure 3 (d) as to leave a transparent area through the article 6 which is in agreement on both surfaces of the film 36, in a manner similar to the process described with respect to Figures 1 and 2. The present invention provides advantages, which are capable of add a transparent window to printed items at any time after they have been printed initially, if for example, a design change is required, and it is possible to add transparent marks or windows it is of variable forms or designs, for example serial numbers for banknotes, in different or consecutive printed articles easily, at high speed and low cost, in addition, the provision of the agreement of the front with the back on both surfaces of the film after the ablation process has been performed it makes printed articles particularly difficult to copy by potential counterfeiters. The application of the ablation process in the manner described above to the articles will advantageously give great problems to the potential counterfeiters in such a way that they will have to reproduce the intricate windows, numbers, etc., which are in perfect agreement on both sides of the article. This invention is particularly applicable to provide windows in bills or other security documents that require a unique serial number or identity number. A window can then represent each digit in the number, and by using appropriate technology, each ticket or security document can have such a unique number. The invention is equally applicable to provide batch codes, manufacturing dates, issue dates, signature or expiration dates, etc. as required. It will be appreciated that various modifications can be made to the embodiments described above without departing from the scope or spirit of the present invention. For purposes of this specification, including the claims, the term "comprises" shall be taken to mean "includes".

Claims (38)

1. CLAIMS 1. A method for marking an article, the article comprises a substrate with opacifying layers on the opposite surfaces of the substrate, the method is characterized in that it comprises: irradiating an area of the opacifying layer on a surface of the substrate with laser radiation of such so that the area of the opacifying layer on such a surface is ablated by the laser radiation to produce a mark by removing the area of the opacifying layer on such a surface, where the laser radiation travels through the substrate and creates ablation in the area of the opacifying layer on the opposite surface of the substrate to produce a marking by removing the area of the opacifying layer from the opposite surface of the substrate.
2. 2. A method for creating a transparent window in an article comprising a transparent substrate with opacifying layers on the opposite surfaces of the substrate, the method is characterized in that it comprises: irradiating an area of the opacifying layer on a surface of the substrate with laser radiation such that the area is ablated by laser radiation to remove the area of the opacifying layer from such a surface, where the laser radiation travels through the substrate and creates ablation in an area of the opacifying layer on the opposite surface of the substrate to create a transparent window in the substrate.
3. The method according to claim 1 or claim 2, characterized in that the opacifying layers on the opposite surfaces of the substrate have higher absorption characteristics for the laser radiation than the substrate.
4. 4. The method according to claim 3, characterized in that the wavelength in the laser radiation is selected in such a way that the opacifying layers absorb and undergo ablation by the radiation and the substrate is substantially unaffected by the laser radiation. .
5. The method according to any of the preceding claims, characterized in that a continuous beam of laser radiation is directed to an area of printed matter on such surface of the substrate.
6. The method according to any of claims 1 to 4, characterized in that at least one pulse of the laser radiation is directed to the area of the printed matter on such surface of the substrate.
7. 7. The method according to claim 6, characterized in that a single pulse of laser radiation is directed to the area of the printed matter on such surface of the substrate, such pulsation has sufficient energy to create ablation on the printed matter on such surface and then travel through the substrate to create ablation on the printed matter on the opposite surface of the substrate.
8. The method according to claim 6, characterized in that a plurality of pulses of laser radiation is directed to the area of printed matter on such a surface of the surface.
9. The method according to any of the preceding claims, characterized in that the substrate is formed of a transparent plastic material.
10. The method according to claim 9, characterized in that the transparent plastic substrate comprises a film of transparent polymeric material.
11. The method according to claim 9 or claim 10, characterized in that the substrate comprises a sheet of two or more layers of transparent polymeric material.
12. The method according to any of claims 9 to 11, characterized in that the substrate is formed of any one or more of the following materials: polyethylene (PE); polypropylene (PP) or polyethyltetraphthalate (PET).
13. The method according to any of the preceding claims, characterized in that the opacifying layers on the opposite sides of the substrate comprise printed matter.
14. 14. The method according to the claim 13, characterized in that the opacifying layers of the printed matter are formed of opacifying inks.
15. 15. The method of compliance with the claim 14, characterized in that the opacifying ink layers comprise a pigment dispersed in a binder of crosslinkable polymer material.
16. 16. The method according to claim 15, characterized in that the pigment comprises titanium dioxide pigment.
17. 17. The method according to claim 15 or claim 16, characterized in that the binder comprises a resin with a polyurethane base.
18. 18. The method according to any of claims 15 to 17, characterized in that the opacifying ink layers include silica.
19. The method according to any of the preceding claims, characterized in that the wavelength of the laser radiation falls substantially within the range of 0.5 to 20 microns.
20. 20. The method according to claim 19, characterized in that the wavelength of the laser radiation is about 1.06 microns.
21. 21. The method according to claim 19, characterized in that the wavelength of the laser radiation is about 10.6 microns.
22. 22. An article that is characterized in that it comprises a substrate having opacifying layers on the opposite surfaces of the substrate, the substrate is formed of a material which is substantially transparent to the laser radiation of a selected wavelength, wherein each of the surfaces has a mark formed within them, such marks are formed by removing a first area of opacifying layer on a surface of the substrate creating the alloy of the first area with the laser radiation of the selected wavelength, and allowing the Laser radiation passes through the substrate to remove one area of opacifying layer on the other surface remaining in accordance with the first area removed from the opacifying layer on that surface.
23. 23. The article according to claim 22, characterized in that the opacifying layers on the opposite surfaces of the substrate have higher absorption characteristics for laser radiation than the substrate.
24. 24. The article according to claim 22 or claim 23, characterized in that the substrate is formed of a transparent plastic material.
25. 25. The article according to claim 24, characterized in that the transparent plastic substrate comprises a film of transparent polymeric material.
26. 26. The article according to claim 24 or claim 25, characterized in that the substrate comprises a sheet of two or more layers of transparent polymeric material.
27. 27. The article according to any of claims 22 to 26, characterized in that the opacifying layers on the opposite sides of the substrate comprise printed matter.
28. 28. The article according to any of claims 22 to 27, characterized in that the article is a security document.
29. 29. A security document characterized in that it comprises a transparent substrate formed of a transparent plastic film having opacifying layers of printed matter on the opposite surfaces of the film, the substrate is formed of a material which is substantially transparent in radiation of the laser of a selected wavelength where both of the surfaces have marks or a transparent window, such marks or windows are formed in the printed matter by removing a first area of printed matter on a surface of the substrate creating ablation in the first area with the laser radiation of the selected wavelength, and allowing the laser radiation to pass through the substrate to remove a second area of printed matter on the other surface, the second area is in accordance with the first area removed from the matter printed on such a surface.
30. 30. The security document or document according to claim 27 or claim 29, characterized in that the opacifying layers of printed material are formed of opacifying inks.
31. 31. The article or security document according to claim 30, characterized in that the opacifying ink layers comprise a pigment dispersed in a binder of crosslinkable polymeric material.
32. 32. The article or security document according to claim 31, characterized in that the pigment comprises titanium dioxide pigment.
33. 33. The article or security document according to claim 31 or claim 32, characterized in that the binder comprises a resin with a polyurethane base.
34. 34. The article or security document according to any of claims 31 to 33, characterized in that the layers of opacifying ink include silica.
35. 35. The article or security document according to claim 22 or claim 34, characterized in that the substrate is formed of any one or more of the following materials: polyethylene (PE); polypropylene (PP) or polyethyltetraphthalate (PET).
36. 36. The article or security document according to any of claims 22 to 35, characterized in that the substrate is substantially transparent to the laser radiation having a wavelength that falls substantially within the range of 0.5 microns to 20 microns.
37. 37. The article or security document according to any of claims 22 to 36, characterized in that the substrate is substantially transparent to the laser radiation having a wavelength of about 1.06 microns.
38. 38. The article or security document according to any of claims 22 to 37, characterized in that the substrate is substantially transparent to the laser radiation having a wavelength of approximately 10.6 microns.